Glass frits used in ceramic enamels are typically low melting glasses. The compositions of the frits for ceramic enamels commonly contain a high percentage of lead oxide, and are lead borosilicate glasses. The lead oxide in the composition is primarily responsible for the low melting point of the frit. Cadmium oxide is also present in a number of ceramic enamels, and is present to provide color stability to enamels containing cadmium pigments. Due to concerns about the toxicity of lead and cadmium oxides, it is desirable to develop frits that do not contain either ingredient.
In addition to having a low melting range (required to avoid deformation of the substrate that the enamels is applied to), frits used in ceramic enamels need to meet other requirements. One of the additional criteria that has to be met is that the thermal expansion of the frit closely matches that of the substrate it is applied to. This is necessary to avoid crazing of the enamel and strength reduction of the substrate.
Another condition that has to be met is that the frit has some degree of chemical durability. This requirement can vary depending on the use of the frit, and includes durability to both acidic and alkaline media. With frit development there is usually a compromise involved with the final properties of the frit. In general frits that have low melting ranges have average chemical durability and relatively high thermal expansions, while frits with high melting ranges have above average/excellent durability and relatively low thermal expansions.
A number of lead free frit compositions have been disclosed.
By way of illustration, U.S. Pat. No. 4,554,258 discloses frits which require the presence of Bi.sub.2 O.sub.3, B.sub.2 O.sub.3, SiO.sub.2 and alkali metal oxides where the bismuth oxide is necessarily present in large concentrations; U.S. Pat. No. 4,376,169 discloses frits which require the presence of alkali oxide, B.sub.2 O.sub.3, Al.sub.2 O.sub.3, SiO.sub.2, F, P.sub.2 O.sub.5, ZnO and TiO.sub.2 and which have critical compositional limits; U.S. Pat. No. 4,446,241 discloses frits which require the presence of Li.sub.2 O, B.sub.2 O.sub.3 and SiO.sub.2 among other oxides; U.S. Pat. No. 4,537,862 discloses frits which require the presence of B.sub.2 O.sub.3, SiO.sub.2, ZrO.sub.2 and rare earth oxides with the weight ratio of ZrO.sub.2 to rare earth oxides being critical; and U.S. Pat. No. 4,590,171 discloses frits which require the presence of Li.sub.2 O, Na.sub.2 O, BaO, Ba.sub.2 O.sub.3, Al.sub.2 O.sub.3, SiO.sub.2, ZrO.sub.2 and F. Reference is also made to U.S. Pat. No. 4,084,976, U.S. Pat. No. 4,224,074, U.S. Pat. No. 4,312,951, U.S. Pat. No. 4,340,645 and U.S. Pat. No. 4,361,654 as additional patents in this general area.
U.S. Pat. No. 5,342,810 discusses zinc-containing lead- and cadmium-free glass frits and their use in forming lead-free enamels that can be stored at below 630.degree. C. Obligatory components in mole percent are 31-50 ZnO, 10-44 SiO.sub.2, 11-35 B.sub.2 O.sub.3 and 11-25 Na.sub.2 O.
U.S. Pat. No. 5,306,674 discusses glass frits that can be fired at temperatures of from about 1100.degree. F. to about 1300.degree. F. The frits have the following composition range in weight percent: 20-40 ZnO, 20-32 B.sub.2 O.sub.3, 10-30 SiO.sub.2 and 4-12 Na.sub.2 O.
Lead free glass frits characterized by low silica content are described in U.S. Pat. No. 5,252,521. The composition range in weight percent includes 35-77 Bi.sub.2 O.sub.3, 10-30 B.sub.2 O.sub.3 and 10-32 ZnO.
U.S. Pat. No. 4,892,847 discloses bismuth based borosilicicates with the following composition range in weight percent: 25-35 SiO.sub.2, 25-45 Bi.sub.2 O.sub.3, 10-25 B.sub.2 O.sub.3, 4-19 R.sub.2 O and 0.3-8 ZrO.sub.2 /TiO.sub.2.
U.S. Pat. No. 5,308,803 discloses glass frits with the following composition range in mole percent: 35-75 SiO.sub.2, 0.1-15 Bi.sub.2 O.sub.3, 0.1-10 Al.sub.2 O.sub.3, 1-30 B.sub.2 O.sub.3 and 5-30 R.sub.2 O.
Additionally, U.S. Pat. No. 4,970,178 discloses lead-free glass frits with good chemical acid resistance having the following composition range in mole percent: 5-14 Na.sub.2 O, 8-25 ZnO, 6-13 B.sub.2 O.sub.3, 45-60 SiO.sub.2 and 0-10 Bi.sub.2 O.sub.3.
It is seen, therefore, that the formulations for the glass frits have varied the nature and concentrations of the oxide components in an attempt to provide acceptable frit formulations. While such frits are alleged to exhibit a variety of desirable properties, they still exhibit deficiencies in one or more performance areas.